text:
Does taxiing time gets considered in a pilot's flying hours?
Basically the title. This could be referenced to US or European rules.

In the US, taxi time can be logged. 14 CFR 61.51 says the pilot must log (emphasis mine):

Total flight time or lesson time.

And 14 CFR 1.1 says (emphasis mine):

Flight time means:
(1) Pilot time that commences when an aircraft moves under its own
  power for the purpose of flight and ends when the aircraft comes to
  rest after landing; or
(2) For a glider without self-launch capability, pilot time that
  commences when the glider is towed for the purpose of flight and ends
  when the glider comes to rest after landing.

Note that in reality, general aviation pilots (I don't know about airline pilots) usually just log whatever is on the Hobbs meter, which may include some time sitting still on the ramp doing post-startup checks, setting up avionics etc. That might not be strictly correct per the regulations, but absolutely no one cares.
This question is closely related.

Note that §61.51 applies when logging toward a certificate. Thus it may be perfectly legal for some GA pilots to not log their time or to log by a different definition. However, pilots flying under §121 and §135 have limits on how much commercial flying they may do in a given period, and this is limited in accordance with the official definition: essentially block to block time.

Airliners don't have Hobbs meters but many airlines use data link to establish block times. Data link automatically sends the "out" time when the park brake is released with all the doors closed, and "in" time when the park brake is set and a door is opened. This doesn't strictly correspond with the legal definition of flight time but most airlines and pilots use this for pay and logbook purposes.  For the aircraft log, the data link also records the airborne and touchdown times which I assume is just weight on the wheels.

@MikeSowsun At first read the automatically recorded block times you describe sound like the §1.1 definition of flight time. But I suppose brake release for the purpose of pushback would precede the time when the "aircraft moves under its own power for the purpose of flight" by some significant amount. That is where the difference lies, right? Do you know of Ops Specs or company GOM that define flight time differently than §1.1? Our GOM defines "flight time" in terms of the start of taxi, so it is essentially in line with §1.1.

Sometimes brake release for pushback can result in a lengthy delay before the engines are started and the aircraft moves under its own power. Airline pilots don't really care about the hours for the logbook but they do pay close attention when those hours determine the size of their paycheck.

It's perhaps worth pointing out that taxi time is only loggable if it's before/after a flight. If you move your plane from one part of the airfield to another that can't be logged.

Yes, taxi time is considered as pilot hours because even though the airplane is not flying actions by the pilot are required. Basically, the US rule is in FAR 61.51

(e) Logging pilot-in-command flight time.
(1) A sport, recreational, private, commercial, or airline transport
  pilot may log pilot in command flight time for flights-
(i) When the pilot is the sole manipulator of the controls of an
  aircraft for which the pilot is rated, or has sport pilot privileges
  for that category and class of aircraft, if the aircraft class rating
  is appropriate;

The "sole manipulator of the controls" is the main thing here, and there's 2 interpretations of this for logging time that I know of, one is that you log from engine start to engine shutdown, the other is that you log from when the airplane starts to when it stops moving (ie. brakes off to brakes on). Both scenarios include taxiing.
I can't find the UK regulation but that's pretty much universal from what I've seen.

Sorry, my downvote, didn't get around to addressing it till now. The first sentence is good. However, the definition of flight time is given in §1.1, as @Pondlife's answer correctly points out. The PIC as SMotC paragraph you quote from §61.51 really has little or nothing to do with the matter. Pilots should, may, or must (which depends on the situation) log—or otherwise keep track of—all applicable flight time, whether it is PIC as SMotC, PIC not as SMotC, SIC, CFI giving dual, or any other type of loggable flight time. Flight time is as defined in §1.1, and that what is or can be considered.

This adds nothing.  I could have sworn I just stated that above.

EASA
Easy Access Rules for Aircrew AMC1 FCL.050 Recording of flight time states:

(g) Flight time is recorded:
(1) for aeroplanes, touring motor gliders and powered-lift aircraft, from the moment an aircraft first moves to taking off until the moment it finally comes to rest at the end of the flight;
(2) for helicopters, from the moment a helicopter’s rotor blades start turning until the moment the helicopter finally comes to rest at the end of the flight, and the rotor blades are stopped;
(3) for airships, from the moment an airship is released from the mast to taking off until the moment the airship finally comes to rest at the end of
the flight, and is secured on the mast;

So yes, taxi time is also considered a flight time in logbook.

Yes, in most cases, taxi time is considered to be part of a pilot's flight time.
Pondlife's answer does a very good job of giving the official definition of flight time as found in 14 CFR 1.1. I want to expand on what that definition means practically in terms of what pilots (under FAA jurisdiction) legally may, or legally must, consider flight time.

When is taxi time considered part of flight time?
For the purpose of clarity, I am going to quote §1.11 again:

Flight time means:
(1) Pilot time that commences when an aircraft moves under its own power for the purpose of flight and ends when the aircraft comes to rest after landing;

I am going to make the assumption that any time spent taxiing is time spent with the aircraft moving under its own power.2
When time spent taxing is for the purpose of flight (for example: starting up, taxiing to the runway, and taking off on a flight), this time is considered part of the pilot's flight time. Since that time spent taxiing is time spent with the aircraft moving under its own power for the purpose of flight, it meets the definition of flight time as defined in §1.1.
When time spent taxing is not for the purpose of flight (for example: starting up, taxiing to the fuel pump, and shutting down to fuel the aircraft), this time is not considered part of the pilot's flight time. Though that time spent taxiing is time spent with the aircraft moving under its own power, it is not for the purpose of flight and does not meet the definition of flight time as defined in §1.1.

Why does this matter?
A pilot may want to keep track of flight time for many reasons, whether toward a pilot certificate, a flying job, a better insurance rate, or merely for personal gratification. Having the agreed-upon definition of flight time as given in §1.1 is helpful for all these ends. However, the important reasons are those where keeping track of flight time is legally required.
In several cases, pilots must keep track of flight time by legal requirement. In some cases, this involves logging the flight time; in other cases this means knowing a record of certain types of flight time with no explicit requirement for written record keeping.
A pilot must document and record (or, log, to put it simply) flight time that is used to meet the requirements for a certificate, rating, or flight review. This requirement is given in 14 CFR 61.51. Since §61.51 does not define what flight time actually is, we have to go back to §1.1 for that definition. In this case, the pilot could log the taxi time if he or she chose to use it to meet the requirements for a certificate, for example.
Pilots flying for Air Carriers or certain Commercial Operators may be subject to flight time limitations. For example, pilots flying under §135 are limited to a certain number of hours of commercial flying. See, for example, 14 CFR 135.267 where a pilot member of a two pilot crew would be limited to 10 hours of commercial flying during any 24 consecutive hours (let's call this "10-in-24"). This commercial flying would include flight instruction, commercial flying for compensation under §91, and other §135 flying, etc. In this case, whether the pilot chose to log any of the above flight time or not, flight time spent taxiing for the purpose of commercial flight would—by legal definition—be included toward that 10-in-24. Let me emphasise this, since it is important to this subject: the totality of commercial flight time—including taxiing for the purpose of flight—is by the legal definition of §1.1 considered to be flight time. Furthermore, since such a pilot is prohibited from accepting any flight assignment that would exceed that 10-in-24 (see more on why in 14 CFR 135.263), the pilot must at least be aware of his or her running 24 hr commercial flight time total, if not logging or otherwise documenting it.
If flight time is tracked either according to a Hobbs time that includes ground time (such as one that starts and stops with oil pressure) or according to block to block time including pushback, the pilot should not have to worry about compliance with flight time limitations since these meet or exceed the definition for legal flight time. However, if flight time is tracked according to either a Tach time or a Hobbs time that excludes some or most time on the ground (such as one that starts and stops according to a weight on wheels switch), the pilot is at risk for non-compliance since these do not record the full legal flight time.

1 Note: I have only included that portion of the definition for flight time that is pertinent to this discussion, since gliders without self-launch capability do not taxi.2
2 I don't think I am wrong about this, but if I am, I would love to be set straight and learn something new!

Note: This answer only addresses operations under FAA jurisdiction.

Yes. 14 CFR 61.51.e.1.i states a sport, private, commercial, airline transport pilot may log flight time whenever the pilot is the sole manipulator of the controls of the aircraft for which the pilot is rated, or has sport pilot privileges for that category and class of aircraft, if the aircraft class rating is appropriate, or when ii)  The pilot is the sole occupant of the aircraft.  The regs make no distinction between flying the aircraft and ground operations.

This is true insofar as §61.51 is quoted or paraphrased. However, as @Pondlife's answer correctly points out, the definition of flight time is given in §1.1. This §61.51 reference regarding PIC as SMotC or PIC as sole occupant does not adequately address the issue of what can be considered flight time. Pilots should, may, or must (which depends on the situation) log—or otherwise keep track of—all applicable flight time, whether it is PIC as SMotC, PIC not as SMotC, SIC, dual given, or any other type of flight time. Flight time is as defined in §1.1, and that is what is or can be considered.

No, you didn't ask for my opinion; I was giving you the courtesy of letting you know why I have downvoted this answer. Your "Yes" is correct, but the reasoning behind it that you offer is not. The definitions of §1.1 are what are important in regards to the meaning of flight time. §61.51 gives additional types of flight time, but does not address whether the flight time in question includes time spent taxiing.

Did he ask about hours logged solely for the purpose of ground ops?

The OP asked about whether time spent taxiing could or would be  (original wording is "gets") considered part of flight hours.

Which is covered under 61.51.

Let us continue this discussion in chat.

I disagree. §61.51 addresses when flight time may be logged as solo flight time, PIC flight time, SIC flight time, etc. §61.51 does not define what this flight time is. §61.51 does not inform us whether or not time spent taxiing is or can be considered part of that flight time. Only §1.1 does that.

In our flying club we charge by TACH not Hobbs.  Some people log hobbs time, but I record the actual engine start on my kneeboard, and then the shutdown time.  So the flight time is the wall clock time for the flight.
This can come into play with student operations where there are minimum experience requirements.  TACH time is based upon rotations of the engine, and Hobbs time is normally based upon the time the oil pressure is up on an engine. Student experience is based upon logged time.
Since not every aircraft has a working Hobbs meter, and I fly all kinds of different aircraft, logging the start time and stop time gives me accurate data for log entries. Practically, motion begins right after start, because the aircraft is pulled away from the tiedown as soon as the oil pressure comes up. Similarly, one normally stops the aircraft, and begins a pre-shutdown checklist, which is normally complete within just a few seconds of engine shutdown.

My instructor always taught me to "test the brakes" immediately upon startup, therefore moving the aircraft under it's own power (and stopping again) so that technically, the hobbs time is an accurate loggable time.. Also a good habit to get into regarding testing brakes before getting distracted with the checklist and having the plane move itself accidentally while you're head down..

Hi, Welcome to Aviation.SE! Answers are generally expected to stand on their own and, thus, fully answer the question here. While your answer implies the answer to the question (in regards to it depending on when the airplane begins moving,) it would be better received if it stated this explicitly (and possibly cited a source.) For more information about how things work on this site, please see [answer], our [tour], and the [help].


text:
Does the FAA advise or require pilots to wear eyewear designed to protect them from lasers?
With several instances of pilots being subjected to laser lights being shone on them, I wonder if the FAA is actually doing something about it.
It's easier said than done to stop people on the ground from shining their laser lights. Maybe a preventive strategy is better. With a host of eyewear available which minimise the risk of laser light, I am curious as to what the FAA and other similar authorities are doing about it.

Similar Question Here

I'm not aware of FAA (or any other regulatory body) recommending protective eyewear for lasers. FAA safety Advisory doesn't include them. There are a couple of problems with using protective eyewear for lasers:

The eyewear is usually wavelength sensitive; The pilot has to wear the correct eyewear that blocks the incoming laser beam; he/she should know the wavelength of incoming laser.
The protective eyewear filters (atleast most) of the incoming light in that wavelength. This may affect the normal sight of the flight crew.

Due to these reasons, the regulatory authorities are more concerned with procedural changes that protective wear. From Boeing's protection against lasers:

... however, airlines should consider the drawbacks that are associated with them. Filtering light reduces the total amount of light entering the eye, which can adversely affect normal viewing, ... . In addition, filtering green light can remove some green flight symbology on flight deck displays and change the appearance of some of the other colors used. As a result, protective glasses should be used with care.

Wavelength sensitivity isn't a problem in the way that you suggest. Green laser pointers (almost) all emit at 532nm wavelength, because that's how the physics works out.

What I've heard the industry is looking into is adding a protective coating for the common wavelengths to the windshield, to protect against these strikes. Dunno how far it's come though

@DavidRicherby You're correct about that; But what if some other color is used - like red(~650nm) or blue (~440 nm). I think green is the most used laser right now (I could be wrong though), but in future high powered lasers of other colors may become available.

source: Stack Exchange

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sort: votes


text:
Is there any rule in commercial aviation, that two pilots in a plane cannot be a married couple?
Is there any rule followed or imposed by commercial airlines that the pilots of a particular plane may not be a married couple? My thinking comes from common workplace rules, where generally HR does not allow a couple to work in the same department, for obvious hierarchical issues.

Common comment: which jurisdiction? FAA? EASA? Else where?

@vasin1987 In this case I doubt it matters. I'm fairly certain that there are no legal regulations on this (though I would be happy to be proven wrong). If there are, it would be interesting to know about them, regardless of jurisdiction. But, I'm guessing that this question will have more to do with airline company rules, not legal.

@JonathanWalters: Your premise is in alignment with my thoughts.. Thanks for clarifying it..

The Air Force, at least when I was flying there, had strict rules forbidding a husband and wife flying as crewmembers in the same aircraft.  Generally, airlines in the US don't have to have any similar policy, and there are plenty of examples of married couples (either pilot-pilot, pilot-flight attendant, or flight attendant-flight attendant) working a flight together.
An individual airline might be able to create such a policy themselves (although I have no idea if there would be legal grounds for a married couple to challenge the rule), but at least in the US there's no legal requirement that prohibits it.

Does this rule apply to ANG too?. There seems to be a huband-wife team (or atleast was).

@aeroalias Dunno about the ANG, although that article never actually said that this couple flew together, just that they were the only married couple in that squadron. One could read it and assume that they do fly together, but if you start out doubting that they can, nothing in the article actually contradicts that idea either.

@RalphJ: I was referring to pilot-pilot situations in commercial aviation..Thanks for your inputs..

The Air Force rule makes sense, as it prevents a single accident from leaving orphans, with the added benefit preventing, well, reducing tomfoolery on long flights.

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Can a pilot fly through a restricted area while flying on a Victor Airway?
For example if you look at V 23-165-597, from Seal Beach CA to Ocean Side CA, you will notice that it goes through restricted area for Camp Pendleton (R-2503A & D):

I was trying to understand what is the rule as far as ATC Clearance to enter those Airspace or any other Airspace in general when you are on those Victor Airways.
Where can I find the FAA rules about these Victor Airways?

Short answer: yes, with permission. Whether or not you're on an airway isn't really relevant.
The basic regulations on restricted airspace are in 14 CFR 91.133 and they apply to all aircraft on all flights:

(a) No person may operate an aircraft within a restricted area
  (designated in part 73) contrary to the restrictions imposed, or
  within a prohibited area, unless that person has the permission of the
  using or controlling agency, as appropriate

The AIM 3-4-3 also makes it clear that you can enter restricted airspace (my emphasis):

Restricted areas contain airspace identified by an area on the surface
  of the earth within which the flight of aircraft, while not wholly
  prohibited, is subject to restrictions.

And it has details of how ATC handles IFR traffic:

1. If the restricted area is not active and has been released to the controlling agency (FAA), the ATC facility will allow the aircraft to
  operate in the restricted airspace without issuing specific clearance
  for it to do so.
2. If the restricted area is active and has not been released to the controlling agency (FAA), the ATC facility will issue a clearance
  which will ensure the aircraft avoids the restricted airspace unless
  it is on an approved altitude reservation mission or has obtained its
  own permission to operate in the airspace and so informs the
  controlling facility.

In other words, you can fly through restricted airspace if the controlling agency gives you permission, either directly or via ATC. In the specific case of the Camp Pendleton restricted areas, the Los Angeles sectional says that you should contact LA Center or SoCal TRACON (for R-2503D), and shows that the areas are not active 24 hours per day:

So in theory you can fly through at 3am without asking anyone, but if you're planning a flight that goes through a restricted area you should make sure you get a flight briefing and ask the briefer about that point specifically. And it wouldn't hurt to confirm again in the air before entering, in case the areas 'go hot' suddenly.

I would emphasize that you should always ask before entering a restricted area, not only to avoid potential violations, but because of the heightened risk of mid-airs that exist during military operations.  Getting a number to call is moot if your wing has just been cut in half by a Harrier.

@SHAF -- also, stray bullets/shells/shrapnel are not kind to airplanes, for that matter!  Or, getting a number to call is rather moot if a Mk 82 just put a big hole in your plane.

Camp Pendleton's current R-2503D zone, which extends along the California coastline to the southeast of the former Camp Pendleton MOA, is a special case.
First off, it can only be activated via NOTAM for a maximum of 20 days a year from 0600 to 2359, and a maximum of 90 days a year between 2400 and 0600 local time. So, for 345 days a year, you can fly right through this airspace at 2000AGL or higher, either VFR or IFR under guidance from LA Center. MCB Pendleton's airstrip remains Class D, and the majority of the remaining R-2503 airspace (including R-2503A below 2000 feet) is active 7 days a week from 0600-2359 local time, extensible by NOTAM.
Second, when R-2503D is active, non-participating craft may still gain entry by contacting MCB Pendleton's tower, codename "Longrifle", whose frequency has been released for general aviation. Clearance would be granted case-by-case based on the nature of activities within the airspace at the time; if denied, the detour for all routes through R-2503D is just offshore, within glide distance of land in most cases.


text:
What does ® mean in the Chart Supplement (A/FD)?
What does the circled r: ® mean in the Chart Supplement (A/FD)? You can see it here to the left of SOCAL APP/DEP CON:

According to the Chart Supplement (formerly the A/FD) legend:

APP CON—Approach Control. The symbol ® indicates radar approach
  control

And:

DEP CON—Departure Control. The symbol  ®  indicates radar departure
  control.

The FAA has different approach facilities classifications, a TRACON has radar but others don't. So you should see this symbol when the approach/departure services are provided by a TRACON or other radar facility.

According to this legend for the A/FD, the symbol indicates that the approach/departure control has radar. This means that ATC can more accurately locate, track, and provide guidance to pilots.


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Are red obstacle lights near an airport required to be lit during the day?
What are the FAA Beacon Light Requirements for 80 foot tall High Mast Poles Near an Airport?  Specifically, do the RED beacon lights need to be illuminated 24 hours per day?  Or, do they only  need to be illuminated at night?
Here are examples of Holophane high mast pole installations around the country.
https://www.facebook.com/media/set/?set=a.236398096375560.78767.131284353553602&type=3
https://www.facebook.com/media/set/?set=a.194847107197326.56223.131284353553602&type=3

It's unclear exactly which lights you're referring to (none of the example pictures you linked to are red), but the FAA AC on obstruction lighting says that only structures over 200ft are required to be lighted:

Any temporary or permanent structure, including all  appurtenances,
  that exceeds an overall height of 200  feet (61m) above ground level
  (AGL) or exceeds any  obstruction standard contained in 14 CFR part
  77,  should normally be marked and/or lighted.

However, there are exceptions:

The FAA may also recommend marking and/or  lighting a structure that
  does not exceed 200 (61m)  feet AGL or 14 CFR part 77 standards
  because of its  particular location.

The same document describes the use of red lights:

Aviation Red Obstruction Lights. Use flashing beacons and/or steady
  burning lights during  nighttime

The rest of the document suggests that red lights are for night use and white for day use. The FAA also has a dedicated portal for obstruction evaluation and reporting if you're involved in building or managing potential obstacles.


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Why would banking be a problem for passengers on the Flying-V aircraft?
In this article, the author raises doubts about the viability of this aircraft:

Sadly for the Flying-V, it will probably fail like the blended wing
  body designs we've seen down the years. It's for the same reason, too:
  airplanes bank as they turn. That's not much of a problem in a
  conventional airliner design, where passengers are never that far from
  the plane's central axis. But as you move further out from that
  central axis the effect becomes a lot more pronounced.

credit: Edwin Wallet, Studio OSO
I don't really understand how banking is a problem. The angle for the passenger remains the same, the only difference being an increased of vertical acceleration when the airplane is initiating / retracting the bank.
An apparent simple solution would be to... bank more slowly...
Is there something I'm missing?
Would the vertical acceleration really be a concern for passengers?

Maybe it's the amount of movement that would be felt as up & down motion. So instead of maybe a foot or two around a central axis, it could be tens of feet as you move farther away from the centerline.

@CrossRoads that's the vertical acceleration I'm talking about yes

Suppose you bank sharply enough - say to avoid a collision - that the acceleration on the passengers furthest from the centerline is greater than 1 g, and they're not wearing seat belts...

@jamesqf How many times a year does an aircraft bank sharply to avoid a collision ? This isn't even a remote concern.

I mean no disrespect to the author, but the blurb underneath the article says "[Author] is the automotive editor at Ars Technica, covering all things car-related." As far as I'm concerned, he does not actually know what he's talking about. Other answers on Aviation.SE about BWB designs will point you to much greater and fundamental challenges.

Avoiding collision is always unpleasant for the passengers, be it in an aircraft, a bus, or on a train. Another problem often brought up with blended wing designs is the time to evacuate. Could the V-design be helpful in this regard?

I think the actual acceleration forces are less of an issue than the motion sickness they would induce.  Sitting on the outside could feel like you're on a roller coaster without the same visual cues.

@jamesqf: And that (among other things, such as turbulence, or a flight-control malfunction) is why you're strongly encouraged to keep your seatbelts fastened even when the FASTEN SEAT BELTS light isn't on.

@Antzi: How many times a year does a commercial jet airliner land on water?  Yet they are required to be able to handle ditching, have life jackets & evacuation slides that become life rafts, &c.

@jamesqf Sure, but passenger comfort isn't really considered for this emergency edge case.

@Antzi: I was thinking less of passenger comfort than of passenger injuries caused by abrupt banking maneuvers.

The author over at Ars is right with being concerned about this concept, but for the wrong reasons.

There are two possible sources of discomfort for passengers sitting farther away from the aircraft's longitudinal axis during a roll manoeuvre:

At the start and end of the roll manoeuvre, the angular acceleration will be felt as a stronger vertical acceleration, as you pointed out (vertical as in perpendicular to the cabin's floor).
Between the start and the end, the aircraft will be rolling at constant angular speed (or roll rate), so the angular acceleration will be zero, but the passengers will still be moving along a circular trajectory (around the aircraft's longitudinal axis) and therefore will also be subject to a centrifugal force (parallel to the cabin's floor) that grows proportionally with the distance from the longitudinal axis.

The roll manoeuvre itself may only last a few seconds, but if the above effects are strong enough, it could be enough to make some passengers feel properly queasy, especially if it's repeated a few times as during departure and arrival procedures.
As you suggest, rolling more gently and more slowly would limit these effects. However, instrument flight procedures always involve standard rate turns, with turn radii expected to remain within certain limits, depending on the aircraft's speed. Rolling more slowly would increase the aircraft's overall turn radius and could make it harder or impossible to comply with currently published procedures.

In this case, standard rate turns would be unaffected: rolling more slowly would just add a small delay to the beginning of the turn.

Yes, which means that the aircraft would take up more space to achieve the same change of heading - that's what I meant with 'overall turn radius'

It's not that bad. A normal roll might take 3s and peak at 15°/s. For a total 30° bank it follows the angular acceleration would peak at 15°/s². From the picture I'll guess the outer passengers may sit around 15m off the axis, which yields maximum vertical acceleration ~0.4G and lateral acceleration ~0.1G. Noticeable, but not too bad. Would be worse for bigger aircraft, of course.

I have a doubt if the question addresses and actual problem. Banking in airplanes is supposed to match centrifugal forces, same as roads and rail tracks are not horizontal in turns. A lateral pull of gravity, or of the force towards airplane floor, when turning around the main fuselage axis won't be noticeable for passengers, as otherwise, structural loads on airplane would be too high, an airplane is not a rollercoaster, an airliner is not an aerobatics machine, not a combat airplane in a dogfight, but perhaps the inertia, momentum forces in having the lot of weight passengers and airplane structure turning will impose an slow banking rate, a regular airliner has most weight close to center, a 'V' shaped machine, or other lifting bodies, do have an important width in weight distribution; ordinary airplanes have just the light wings quite apart from center line. Is this a reasonable answer to issue? Thanks. Salut +

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text:
What is this turn called and how is it performed?
I've found pictures and videos of jets rotating 90 degrees so that one wing points straight down to the ground and the other is straight up, then they suddenly turn away from their course while remaining in that position.
Here's a video of that maneuver (link goes to 2:10).
What are the aerodynamics that can make this kind of turn possible? In a lot of YouTube videos this type of turn happens after aerial refueling. Is this type of turn possible on a large airliner, and if it is, when do pilots of this airliner ever use this method in flight?

The maneuver is called a "breakaway maneuver" or simply a "break", it's also sometimes colloquially referred to as "peeling off" in the context of aircraft leaving formation. You can see it more clearly in this video:

This is a standard maneuver used when disengaging from a intercepted target, or as a way to safely exit formation flight ensuring that the departing aircraft is clear of the rest of the formation group.
It also has tactical advantages as seen in the photo you posted (when being tracked by an infra-red/heat-seaking missile, dropping flares and a hard breakaway is one technique used to convince the missile that the flare is actually your engine, by getting your real heat source out of its "line of sight" quickly).

Aerodynamically the breakaway maneuver is nothing more than a steep turn: Roll the aircraft into the desired bank, setting power and pitch up to maintain your altitude and airspeed in the turn as desired.
A Cessna 152 flying in formation could break away using the same maneuver, though the bank angle and airframe loading (G forces) would need to be maintained within the aircraft's limitations.
The primary difference with fighters and attack aircraft is that the bank angle is dramatically steeper (in the extreme, a 90-degree "knife-edge" bank) and the turn much tighter because these aircraft have the excess power (thrust) required to sustain flight in that configuration and are sturdy enough to withstand the G loading induced by a "hard pull" in the breakaway maneuver. The abrupt wings-up turn along with the nice roar you get advancing the throttles makes the maneuver an impressive one for airshow crowds to watch.

Could you develop a bit how the altitude is maintained while the wings are rolled 90°, and the vertical component of the lift seems null? Is that possible with the lift of the fuselage, or some rudder deflection?

@mins Much like in a straight vertical climb thrust is what's keeping you in the air in a knife-edge turn, along with excess energy (airspeed) you may have from whatever maneuver lead you into the turn. How long you can maintain altitude in that configuration depends on how much excess energy you have to make up for the fact that your regular lift vector is no longer pointing "up" - a Cessna 152 couldn't do it for more than a few seconds (if at all), while an F-16 could sustain knife-edge flight much longer.

Thanks. I understand that can work only if the longitudinal axis is not horizontal and the thrust is directed a bit up upwards like this (Youtube).

The angle of bank in a level turn is less than 90 degrees. It can get real steep, but if t was 90, the nose would slice down.

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text:
How long is the horizontal stabilizer on a Boeing 737?
I can never find information about the length of the horizontal stabilizer all I can find is the height of the vertical stabilizer.

According to this Boeing pdf (page 39), the total length of the horizontal stabilizer on a 737-800 is 47 ft 1 in.

Total wingspan or the length of each horizontal stabilizer.

Total length of the entire horizontal stabilizer. Looking at the diagram, the space between each stabilizer (at the tail) is negligible, so you could divide by 2 for a rough estimate.


text:
How does a fighter jet perform good and quick maneuvers with such small wings?
Fighter jets like the F-16, F-15, etc. have wings smaller than the length of the fuselage and can perform great maneuvers very quickly while an airliner with a long wingspan takes a lot of time to perform these kind of maneuvers. How does a short wingspan on a fighter jet perform such great maneuvers?

Some fighter jets have a long winspan. The ones I am reffering too are the f15,f16,f18, and the f35.

Jet fighters have low aspect ratio wings.  Their wing area is not as small as you'd think from just looking at wing span.  Even so, they get by with small wings.  See my answer, and https://en.wikipedia.org/wiki/Aspect_ratio_%28aeronautics%29.

Fighters have actually larger wings than airliners - when compared to their mass (smaller wing loading).

"If I were meant to fly at speed, I'd have a falcon's short wings, and live on mice instead of fish." -- Jonathon Livingston Seagull, Richard Bach

Fighter (combat) aircraft are designed to perform quick maneuvers in order to get into a position quickly to engage and shoot down an enemy aircraft or evade incoming threats (like missiles, etc.). Civil aircraft like airliners are designed with other things in mind like comfort, safety, reduced fuel consumption, etc.
The wings are only a part of the story in performing maneuvers like those performed in air combat maneuvering, which depends on a number of aircraft characteristics like,

Power - For carrying out most of the maneuvers (like wingover), the excess power available matters. Fighter aircraft have more excess power, which translates into more acceleration. That allows it it enter into maneuvers quickly compared to the civil aircraft. In case of emergencies, the combat aircraft can use their afterburners to gain significant extra power.
Thrust-to-Weight Ratio - Most of the modern fighters have high thrust-to-weight ratios, usually more than 1 (which allows them to accelerate in vertical climb). This allows the fighters to accelerate far faster compared to the civil aircraft. For example, F-15 (>1) had a thrust-to-weight ratio more the three times that of the Concorde (~0.33).

Source: strategypage.com

Stability Most of the fighter aircraft today are designed to be unstable. This makes the aircraft respond to control inputs much more quickly compared to the airliners (which are usually not unstable in the roll axis). This means that the combat aircraft are capable of maneuvering much more quickly.
Inertia In case of large wings, the roll inertia is very high. This prevents them from performing quick maneuvers. Also, the damping of the large wings is also to be considered. In case of fighters, however, the short wings, having smaller inertia helps in achieving high roll rates.
Wing Loading The smaller the wing loading, the better the turn performance. Most of the combat aircraft employ a blended wing-fuselage design, that reduces the load on the wings. For example, the Eurofighter Typhoon (~300 kg/$m^{2}$) has about half the wing loading of A380 (>600 kg/$m^{2}$). In fact, an F-15 produced enough lift with half a wing gone to perform a successful landing.

Source:www.armchairgeneral.com

Control Surfaces- In general, the combat aircraft have more number of control surfaces like canards (in addition to primary control surfaces) compared to the civil aircraft. This enables the pilot to maneuver the aircraft much more quickly compared to airliners.

Source: www.blokeish.com
Due to these reasons, the fighter aircraft are able to perform maneuvers much better and faster compared to airliners.

Re the instability: it may be worth noting that in a lot of fighter jets, the instability is so great that you have to constantly make micro-adjustments just to maintain level flight -- whereas non-fighter planes are often stable enough that if you just let go, they pretty much stay where they're going. These micro-adjustments are why fighter planes have to be fly-by-wire; a human could never make them fast or precisely enough. The F-16 was the first production plane to use this system.

No answer mentions it,  but the vectored thrust helps too.

Yeah mins; I was looking through the comments to find this. It allows the plane to change the angle of its exhaust. Here is the wiki link https://en.wikipedia.org/wiki/Thrust_vectoring

Can anyone comment on what 2D versus 3D TVC (Thrust Vector Control ?) means in the last figure? Being a nitpicking scientist type, I would think it is either 1D (just up/down or just left/right) or 2D (both up/down and left/right). I don't understand how you can get 3D.

In your scenario you're forgetting the initial thrust coming out of the port towards you on the z axis? BUT here it refers to pitch, yaw and roll, in relation to the crafts "degree of freedom".
Edit: link to answer in site http://aviation.stackexchange.com/questions/16268/what-is-2d-and-3d-thrust-vectoring

@yshavit : Indeed. Just an example of the opposite extreme: I once piloted a German glider which had the instruction for most of the emergency cases, including stall and spin, to just let go of all controls and wait for the aircraft to automatically resume level flight. This was solved purely mechanically, by making the aircraft so stable that it was extremely hard to enter into a spin even intentionally.

Wingspan and maneuverability are not necessarily closely linked. Important factors that affect maneuverability are mass and thrust, which can be combined into a thrust to weight ratio, and also wing loading, which does relate to the size of the wings.
The thrust to weight ratio of a 737-500 is around 0.32, for an F-15 it can exceed 1.1, which is a huge difference (larger is better).
An F-15 has a nominal wing loading of 358 kg/m², compared with 497.05 for a 737-500 (lower is better).
The combined effect of increased thrust to weight ratio and reduced wing loading means a fighter jet is a very different kind of plane from a passenger liner.
Finally, regarding wing loading, on many fighter jets starting in the 1970s, the body also partly acts as part of the wing area. Notice that modern fighter fuselages do not have the cylindrical or ellipsoidal cross-sections that you see on a passenger liner. This design style further separates wingspan from maneuverability.

Fighters and aerobatic aircraft have large control surfaces (ailerons, elevators, rudders, elvons, stabilators, etc) as it is control surfaces and not the wings themselves which cause the attitude changes which maneuver the airplane.

To be most clear, fighters have larger control surfaces relative to their overall mass and cross-sectional areas. It is possible to have absolutely smaller control surfaces that are more effective than larger ones because of the relationship to the rest of the plane.

Changing attitude is only one part of the story.  If your wings don't provide enough lift to change your direction, changing attitude without changing direction leads to stalling.  See my answer.

Lift is proportional to wing area, not just wingspan.
Fighter jets typically have narrow wings (as you noted), but they run most of the length of the fuselage (low aspect ratio).  Larger slower-moving planes typically have long skinny wings.

Low aspect ratio wings are usually used on fighter aircraft, not only
  for the higher roll rates, but especially for longer chord and thinner
  airfoils involved in supersonic flight.
  -- wikipedia's Aspect Ratio article

Many jet fighters, esp. the F15 as noted in several answer, generate lift from the fuselage, significantly increasing effective wing area.  The entire wingtip-to-wingtip span is wing, because there's no non-wing fuselage in the middle.

So even though the premise of the question is somewhat flawed, there are reasons:
I'm not sure if fighter jets have more or less wing area per mass than larger craft.  It's reasonable to assume they have less area per nose-to-tail length, though, because mass increases with the size3, while surface area increases with size2.
Compared to a big plane, 1/2 length -> 1/8th mass, requiring only 1/8th wing surface area, not the 1/4 area you'd have from a proportional scale-model.

To turn, you need to get the entire mass of the plane moving in a different direction.  To pull into a loop, you not only need to change attitude quickly (large control surfaces); you also need lift from the increased angle of attack to change the plane's motion vector.  (With large control surfaces but not enough lift, you pitch up but keep moving horizontally, and stall).
"Enough lift" depends on the mass of the plane, because $F = m a$.  Keeping proportions the same, a larger plane would have less lift per mass, because of the cube vs square issue.
Another important factor is speed.  The faster a plane is moving, the more extra lift for you gain from pitching up.  The faster you're going, the more air you can push on per wing area.  At high speed, you don't need as much wing area to produce the max ~9G of acceleration a pilot can handle.
In terms of turning-radius, this is more than cancelled out by the centripetal force needed for a constant-radius turn increasing quadratically with speed.  (Thanks @Todd for catching this).  Degrees-per-second (angular velocity, ω) is similarly not helped by moving faster, once you're going fast enough to make a max-G-force manoeuvre.
F = lift-factor * v = ma.
$m \omega^2 r = mv^2/r = F$
$\omega^2 r = v^2/r  = F/m = a = 9G$
$\omega^2 = v^2 / r^2$
$\omega = v/r$.  But for constant $a$, $r$ is proportional to $v^2$.
$\omega = v / (v^2/a) = a/v$  (where $a$ is constant)
So at speeds fast enough for max acceleration to be the limiting factor, turn rate ~= 1/v.  At lower speeds, where achievable $a$ increases ~linearly with speed, ω is about the same at any speed up to $a = 9G$.  High thrust is needed to overcome the high drag of high lift / high angle-of-attack turning.
Small planes also make it easier to make wings strong enough to not snap them off at a high angle of attack (difference between heading and facing, whether it's in the vertical plane, or turning horizontally after rolling to near 90 degrees.)  Low aspect ratio wings spread the load over a longer attachment point with the fuselage, helping with this.
In a high angle-of-attack, the engines are contributing some of the necessary centripetal force to bend the plane's momentum vector, because they're pushing the plane in the new direction, not just along its current trajectory.
So combining all these factors, fighter jets get a lot out of their wings by moving fast, having sturdy wings that can take high loads, and by being light so the wings don't have as much mass to turn.
Appropriately sized control surfaces are obviously a requirement, to hold a figher jet in a high-angle-of-attack turn.
I think vectored thrust contributes mostly in this area.  In a (non-inverted) loop, a jet would have its thrust vectored upward, along with the elevators, pushing the tail down.  This means less thrust is contributing to centripetal force; instead it's helping to hold the plane in a higher angle of attack so the wings can pull the plane in a tighter loop.
I'm sure there are some mistakes here, since I don't actually design airplanes, or even fly them outside of video games.  I'm just applying simple physics and making stuff up.  It looks like a lot of what I said is pretty much what wing loading is.

"The faster a plane is moving, the more extra lift for you gain from pitching up. The faster you're going, the more air you can push on with the same size wings." But also the more extra lift you need since you have more momentum for the same mass. Force grants change in momentum, and the other side of that is greater changes in momentum require greater force. This is why there is a corner velocity below the maximum speed of the plane instead of faster always being better.

@ToddWilcox: Good catch.  Updated my answer (which now probably spends too much time on the math there...).  Maximum acceleration is limited to about 9G, and at high speed you don't need as much wing area to produce that much lift.  That's what I meant to say before, not actually that you can turn faster if you're going faster.

Acceleration = force/mass
Fighters have greater accelration (whether linear or rotational), because their force-to-mass ratio is greater than for an airliner. Simple physics.
For a rotational acceleration, moment of inertia takes the place of mass in the above equation. Shorter wings reduce the moment of inertia.

shorter wings reduce the moment of inertia not only, they also give much less rotational damping. moment of inertia will slow down the acceleration and the damping will slow down the rotation.

"For a rotational acceleration, moment of inertia takes the place of mass in the above equation" and force is replaced by torque. "Shorter wings reduce the moment of inertia" and also the potential torque from having a control surface at greater distance from the axis of rotation. It's not immediately obvious that these two competing factors resolve in a direction that increases, rather than decreases, acceleration.

@DavidRicherby For for given accereation rate (linear or rotational), less force (torque) is required for an object with a lesser mass (moment of inertia). Sure, the fighter's aileron may be creating less overall torque (due to a lesser arm and surface area), and if it is, then the MOI MUST be soooooo much lower than the airliner's that it results in an overall greater acceleration.

@SteveH Sure but now things are backwards. You're deducing the physics from the behaviour of the aircraft (the fighter is more manoeuvrable so the MOI must decrease more than the torque), wereas you're supposed to be using the physics to explain the behaviour.

In the glider community it's been known since the early ages that a longer wingspan decreases maneuverability: gliders with a larger wingspan are more "lazy" with the ailerons.
Of course it is not the only factor, and as wing design has evolved into more sophisticated airfoils, the effect is less noticeable than it used to be in the past.

I can't believe this hasn't gotten an upvote in almost 8 years!  There are some better answers, but this is the very first thing that came to my mind when I read the question, and I don't think any of the other answers give it enough credit.  Sheer physical size is a major contributor.  Consider an RC model of an airliner, it will be much more "snappy" than the real thing.


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How do helicopters hover?
I know that helicopters blades are like rotating wings that generate lift in the same way, but how does this make a helicopter hover?
Other question; how does the tail rotor help with the balance of the helicopter and why does it have to be on the left side of the vertical stabilizer instead of in the center?

Not all helicopter tail rotors are on the side of the vertical stabilizer: Eurocopter EC 135 tail rotor

@GregHewgill Not sure what to call that. I believe it does the same thing with a helicopter like a tail rotor right.

Yes, it does. The general name for that kind of assembly is a ducted fan.

By being so ugly that they repel the Earth.

Not all tail rotors that are not in the centre are on the left side.

I think the second question should be asked separetely

@ManuH Completely agree.

To understand how a helicopter works (and in fact the mechanics of other aircraft too), it's helpful for you to be familiar with a key concept in physics, called Newton's Third Law of Motion. It says, in general terms, "for every action, there is an equal and opposite reaction".
Here's a simple thought experiment. Picture yourself wearing a pair of rollerskates, standing up, holding a basketball. Now, throw the basketball out in front of you using a two-hand pass (bring it to your chest and push it out in front of you). You'll find yourself moving backwards on the skates. You won't be moving as fast backwards as the ball will be moving forwards, because you have more mass than the ball, but you'll move, because the force of you pushing the ball out forward created an equal force moving you in the opposite direction.
A helicopter, and in fact most powered aircraft, work the same way. when a helicopter's rotor moves through the air, it pushes air downwards through a combination of its airfoil design, which moves air downwards as it passes over the top of the rotor, and an upward angle to the blade relative to the oncoming air that pushes the air with the underside of the rotor. By Newton's Third Law, the force pushing the air downward creates an equal and opposing force pulling the rotor, and the helicopter attached to it, up into the air.

To hover, all that's required is for this force pulling the helicopter up (you can consider it as either "lift" or "thrust") to equal the force of gravity on the helicopter, which is its weight. If these forces balance, the helicopter will remain the same distance off the ground. If lift is greater then weight, it will rise into the air, while if weight it greater, it will sink or fall.
Now, normal aircraft have to move forward through the air in order for their wing to get the airflow needed to generate lift. A helicopter, by rotating its wing, can produce the airflow over the lift surface without the entire helicopter having to move forward (or backward or sideways for that matter). So, it can rise straight up into the air and truly hover over a single spot on the ground.

The tail rotor of a helicopter is necessary to counter another force produced by the helicopter; torque. Torque is similar to force - really it is a force - but it's being produced in a circular direction instead of a straight line. The helicopter's engine uses torque to spin its rotor. Again, by Newton's Third Law, if the rotor of the helicopter is being spun clockwise by the engine, the helicopter itself will begin to spin counter-clockwise.
To counteract this torque, most helicopters have a second, smaller rotor which works the same way as the big one, but directs the force of its thrust sideways in the opposite direction to the way the helicopter's body wants to spin.

This sideways rotor doesn't necessarily have to be on the tail, but it works best if it's on the end of a long lever arm, such as the tail boom of the helicopter. The closer it is to the main rotor's shaft, the more thrust it has to produce in order to counteract the torque of the main rotor.
It doesn't have to be on the side of the tail either; many modern helicopter designs have the tail rotor spinning inside an aerodynamic duct in the middle of the tail:

Helicopters also don't necessarily have to have a tail rotor at all; all that's required is equal, opposing torque to that of the main rotor. A common substitute for a tail rotor is a second, counter-rotating main rotor:

You may also mention the X3 and the NOTAR for the non-usual usual anti-torque systems.

The major forces acting on the helicopter are the same as that of the aircraft.

The helicopter wings generate lift which is perpendicular to the rotor plane (the planes in which the rotor rotates).
This lift vector is rotated to the forward to get the thrust.
The weight of the helicopter acts vertically downwards.
The drag is opposite to the direction of movement of the helicopter.

Source:www.danubewings.com
At hover, this lift is simply equal to the weight of the helicopter. There is no forward movement and thrust is equal to zero.
In aircraft, the forward motion of the aircraft creates airflow over the wings, which in turn generate lift. In helicopters, the blades are rotated to generate airflow over the blades. However, the rotation of the blades causes a torque to act on the helicopter, which has to be balanced to prevent the rotation of the helicopter.
There are various ways to counteract this torque. In one of the most widely used methods, introduced successfully by Igor Sikorsky in VS-300, this balancing moment is provided by the tail rotor.

"Heli tail rotor dia" by FAA - FAA - H808321 Rotorcraft Flying Handbook. Licensed under Public Domain via Wikimedia Commons.
There is no need for the tail rotor to be in any particular side of the vertical fin. For example, the Mi-8 had it on the starboard (right) side.

Source: www.military-today.com
Mi-17, which was developed from Mi-8, had the tail rotor shifted to the port side of the vertical fin.

Source:www.forceindia.net
Some helicopters like the EC-145 has the tail rotor in the middle of the vertical fin, an arrangement called the fenestron.

Source: blogs.star-telegram.com
However, the helicopters need not even have a tail rotor for hover, The necessary anti torque can be generated by multiple ways, like:
Tandem rotors in case of CH 47

Source: www.helis.com
Coaxial rotors in case of Ka 50

"Russian Air Force Kamov Ka-50" by Dmitriy Pichugin - http://www.airliners.net/photo/Russia---Air/Kamov-Ka-50/0920728/L/. Licensed under GFDL 1.2 via Commons.
Intermeshing rotors like K-Max

Source: www.ainonline.com
NOTAR system like MD 520N

"Md500n.g-smac.arp" by Adrian Pingstone - Own work. Licensed under Public Domain via Commons.
Hiller HOE-1 used a tipjet

Source: www.aerospaceweb.org

It gets a little more complicated for helicopters than your first paragraph implies. On a traditional plane, thrust and lift are easily separated; the propeller produces thrust, the wing produces lift, and these two forces operate at roughly right angles in level flight. On a helicopter, the lift vector and thrust vector are pretty much the same; there's no dedicated system to produce forward thrust.


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What are some differences that make the new 737 MAX winglets more efficient than a conventional winglet?
I was reading this article about the Boeing 737 MAX final assembly and it was saying something about the new winglet generating lift and increasing efficiency. Can you explain to me how this works, and what other changes to the new winglet are made to make it more efficient?
source:Gizmag

possible duplicates: What winglet airfoils are used on the Boeing B737? Is wingtip design mature? Why are there different types of winglets?

The 737 MAX winglet is the latest develpment in the 'winglet war' going in between the Boeing and Airbus.
As it has been described how winglets work and its advantages, I will not go into those. Most of the wingtip devices in use today are there simply to reduce the effect of wingtip vortices i.e. they are more or less vertical to the wing.
For example, the 737-200 winglets produce little lift.

Source: .b737.org.uk
The same goes for 737-800 variants, which use blended winglets which do produce a little lift (redistributing the spanwwise loading).

"A Garuda Indonesia Winglets Boeing 737-800 NG" by FathirLeone - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons.
And also the 737 NG 'Split Scimitar' wingtips (where do they get these names?).

This adds some weight to the aircraft. For example, the 737 winglet weighs arounf 180 lbs and the total structutal weight increase due to it is around 480 lbs. Obviously, the winglets more than compensate for this increase, that's why we see them used.
With modificaitons, the wintip devices can be used to produce lift and this is the path being taken today with new wingtip device designs, especially with the blended winglet. Boeing took this to the extreme in 787 raked wingtips, which is basically in the horizontal plane, which increased the actual aspect ratio of the wing.

Source: malaysianwings.com
Another idea is to use them to generate lift by changing their angle of installation. This makes the 737 MAX winglet somewhere in between the legacy 737 and 787 winglets. The 737 MAX Advanced Technilogt (AT) winglet has two parts:

An upper part which is at an angle, resulting in some lift being produced.
A lower strake.

The idea is that the winglets also produce some lift in addition to drag reduction. According to Boeing,

The AT winglet further redistributes the spanwise loading, increasing the effective span of the wing. The AT winglet balances the effective span increase uniquely between the upper and lower parts and therefore generates more lift and reduces drag. This makes the system more efficient without adding more weight.

The AT winglets distribute spanwise loading like this:

Source: b737.org.uk
Note: the first one is the baseline aircraft with no winglet. The second one has the 'blended' winglet, while the third one gives the AT winglet. As can be seen above, some of the lift is generated by the winglets itself.

Laminar flow also contributes a bit.

@fooot Boeing says they use something called 'Natural Laminar Flow Technology' in AT winglets. However, it appears that they are talking about surface treatment to reduce skin friction.

Yes, and the "reduce skin friction" part is by allowing more laminar flow across the surface.


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Can an airplane fly with greater weight than lift and how?
I just really don't understand how an airplane can fly with greater weight than lift.

I'm not sure I understand what you're asking here, but your question as it's phrased is the equivalent of asking "If I can lift 400 pounds how can I lift this 500 pound box?" (The answer should be self-evident.)

I don't understand your question fully. Can you give some actual numbers, or a link to some article that explains what you're asking about?

weight 5:4 lift

As described by Peter Kampf's answer in the question @fooot linked, I believe that if the weight of the plane is slightly greater than the lift generated by the wing, it's possible to compensate by increasing the AoA and using excess thrust to generate a lift component. However, it's going to be an expensive flight (fuel consumption) and most likely illegal, since you'd be significantly over certified MTOM for the plane.

@jjack And not only would it only fly downward, but it would be constantly accelerating downward.

Thanks for the answers I get it now thrust addes additional lift that beats the weight.

@reirab: Isn't the situation w > lift the one for a glider? Overall the glider is descending in the air mass, but the ascending air can increase the glider's height / altitude.

@mins No, a glider would have weight = lift for most of its flight regime, just like a powered airplane. It would only have weight > lift when it's accelerating downwards relative to the air mass (again, just like a powered airplane.) f = ma. If there is a net down force, there is a net down acceleration (provided your airplane has positive mass; if your airplane has negative mass it's different and I'd like to co-author a paper with you. ;) )

@reirab: Thanks. Agreed, but it gave me a strong headache. Negative mass: Rudiments for flying saucer science :-), hence would be plagiarism.

There are many situations where the weight is not equal to the lift.
Lift is, per definition, the aerodynamic force perpendicular to the undisturbed incoming airflow.
The weight of the aircraft is due to gravity, hence pointing down.
An example of a steady situation in which the lift is not equal to the weight is the climb phase. During climb the lift is actually less than the weight.
In your comment you mention a weight-to-lift ratio of 5 : 4. If that would be during an unaccelerated climb, the flight path angle would be 37 degrees and the thrust minus drag would have to be 60% of the weight.
The image below shows the weight vector $W$, lift vector $L$, and excess thrust vector $T$ in a ratio 5:4:3.
The combination of the lift and thrust vectors is equal is size and opposite to the weight vector. It is denoted $W'$ in the image.
For simplicity I left drag out of the image and assumed thrust is purely acting in the direction of travel.


text:
Why doesn't GE replace all 4 engines for on their flying test bed instead of just 1?
GE (General Electric) flies Boeing 747s equipped with new engines that are being tested. Why can't they equip the test airplane with all 4 of the new engines? Wouldn't the thrust be imbalanced by replacing only 1 engine? As you can see, it may have 1 big engine and 3 smaller ones:

Other times, smaller engines are being tested. Would the smaller engine provide enough thrust for a large, heavy airplane like the 747?

"on an airplane this size it wouldn't have much thrust because of the weight of the 747." I'm not sure this is what you meant to write. Thrust is not related to aircraft weight. I think you mean to ask if the lower thrust would cause performance problems (compared to the default engines).

Would the smaller engine provide enough thrust for a large, heavy airplane like the 747?  It flies and GE were happy to fly it.  That's probably all the answer you need to that one.

Also note that these 747's are testbed aircraft: likely empty, and almost certainly running fairly low fuel loads. They aren't taking 14 hour passenger flights, so they aren't at maximum take off weight and don't need as much power as an in-service aircraft.

It's perfectly legal to operate a 747 on only 3 engines for purposes of a ferry flight. The restrictions that apply are that it must be carrying no revenue payload and have aboard only the required crew. I would imagine the same rules apply for test flights. Thus there would be no problem operating with a smaller than normal engine thrust-wise. They would, of course, have reduced performance for a given weight.

They only change one, because that one is a new engine that is being tested.
If you are testing a new engine you change 1 at first so if it fails you have 3 others.  If you change all 4 and there is a systematic defect in the design, now you have 0 good engines instead of 3.

You will also see that the test engine isn't always one suitable for a 747.  This is because they may be testing a new design or a new alternative fuel and it is safer to test an unknown with 3 good engines (e.g. a 747) rather than with just 1 good engine (any two-engine airplane).  This increases the safety margin if the test engine fails.

Running a different engine with a different thrust rating requires some thought about asymmetric thrust.  Doing this on a 4-engine airplane simplifies this greatly over a 2-engine airplane.  Ways to accommodate different thrust from the odd engine:

De-rate the new engine so it only produces thrust equivalent to the normal engine
Run the new engine at reduced thrust
If the new engine is inboard, reduce the outboard engine thrust to balance the total yawing moment from that wing with the other side.
Don't worry about it and trim the airplane to compensate.

The other advantage to doing this on a 747 is that a 4th engine is technically not really needed as a 747 can takeoff, fly and land with only 3 operating engines.

Any idea how GE addresses the asymmetrical thrust? That's a good part of Ethan's question, although I imagine they just throttle back #3 to match #2.

@egid Yes. That's what I think they do.

@egid derating would work, as would compensating loadout. Of course the 747 can handle a certain amount of asymmetric thrust without such measures quite well.

Possibly also worth mentioning that these testbed aircraft run fairly light, so there's no real need for 4 engines at all: as such, two engines running on partial power isn't much of an issue anyway and the "partner" engine can be safely throttled back

@egid one way that they are doing that is having it as the inner engine, where too much/little thrust will have less lateral impact than if it was on the outside. Think of the lever arm that the engine thrust is affecting, outside engines have a longer arm than inner ones.

I'd imagine that a modern FADEC could be programmed to provide equal thrust from left to right, no matter the specific thrust available from each engine. i.e. if the new, test engine produces more thrust, the other engine on that side runs at a reduced setting so the thrust is balanced. If the new engine produces less thrust, then the corresponding engine on the other side is automatically run at a lower power level to keep things balanced.

@egid While they probably do normally adjust the #3 engine to match the thrust of #2, that's not always the case. In some cases, they just don't bother to use #3 at all. Or #1. Or #4. :)

@JonStory that is true, but it also isn't that critical.  Even a heavy 747 can fly on just three engines.

They are not testing that the engine works, that's already been proven with hundreds of hours of regulated and rigorous ground running.  They are testing the "on wing" systems and behaviours in various flight conditions.
The on wing systems will also have been tested in ground runs before the first flight.
Therefore, only one engine is needed.

Although you already have a correct answer and it is accepted too but i would add another point to the mix. They are testing a new engine which uses a new technology and is uncertified/unproven. Why should they make 4 of that version and then find out a number of things to fix and go back to all 4 of them first before moving on to produce more models? That costs a lot of money and time.
One is enough.

Wouldn't the thrust be imbalanced?

Even on our R/C Aircrafts we can have different engine RPMs for different engines (on a multi engine model), let alone a Jumbo Jet! They can all operate at different turbine speeds and different thrust levels to balance as and when needed.


text:
Can turboprop blades break the sound barrier?
This question doesn't relate to the actual speed of the aircraft, but of the blades themselves. I have heard helicopters had problems of this happening if the rotors spin too fast it can break the sound barrier, so could this happen to a turboprop?

A minor edit.  If there is one thing that annoys helicopter pilots more than calling them "choppers", it's calling their rotors "wings" ;)

@Simon And, yet, it's technically correct, no? Hence the terms "rotary-wing" vs. "fixed-wing."

The category of aircraft is correctly "rotorcraft",  Rotary wing is an expression which is widely used, but the rotors are never called wings.

Fun fact (related to turbofans rather than -props): the very loud humming noise you hear on take-off inside a jetliner is in fact due to the fan blades breaking the sound barrier.

Can it happen: Yes
Does it happen: Generally no
As a matter of the pure physics propeller tips can go supersonic and some times do. There is at least one case of this being done by design on the XF-84H which was built to be one of the fastest propeller planes.

The main issue it faced was the noise generated by its supersonic prop. It is considered one of the loudest aircraft ever made as its prop was supersonic even at idle speeds. Due to the shock waves generated by breaking the sound barrier great care is often taken to ensure that propellers don't go supersonic.

Quoth Wikipedia: "...[the propeller blades produced] a continuous visible sonic boom that radiated laterally from the propellers for hundreds of yards. The shock wave was actually powerful enough to knock a man down; an unfortunate crew chief who was inside a nearby C-47 was severely incapacitated during a 30-minute ground run.  Coupled with the already considerable noise from the subsonic aspect of the propeller and the T40's dual turbine sections, the aircraft was notorious for inducing severe nausea and headaches among ground crews." (1)

"In one report, a Republic engineer suffered a seizure after close range exposure to the shock waves emanating from a powered-up XF-84H.
The pervasive noise also severely disrupted operations in the Edwards AFB control tower by risking vibration damage to sensitive components and forcing air traffic personnel to communicate with the XF-84H's crew on the flight line by light signals.  After numerous complaints, the Air Force Flight Test Center directed Republic to tow the aircraft out on Rogers Dry Lake, far from the flight line, before running up its engine." (2)

It seems sometimes happens on the Tupolev Tu-95 "Bear" long-range strategic bomber.
According to various sources like this (the first result from a Google search), the tip of the propeller goes transonic while turning at full rpm with a decrease in performances and a severe increase in noise.
The Tu-95 has straight blades, like many old turboprop aircrafts:

To partially overcome this problem, modern turboprops use "scimitar" blades, like the one on the C-130J:

In the same way a swept wing is more efficient at high speed in comparison to a straight wing, at high tip speed, a scimitar blade is more efficient than a straight blade.


text:
Can an engine on a Boeing 737 fly just using compressed air instead of jet fuel?
I was just curious because the air is very compressed in the engine and some things do run on compressed air. And what if the engine was powered all by electricity?

Where does the compressed air come from? Or the electricity? You will need some energy source on board that is not too heavy. Kerosene is best for the time being for its energy/weight ratio, though this does not exclude some people (such as Airbus) from working on small prototypes of electrically-powered planes. Not yet up to big aircraft, though.

You can get thrust out of a balloon… but it'd need to be a pretty big one to lift a 737. Then it'd be more efficient to use it as a blimp then as propellant.

@bjb568 You can get lift out of balloon. It doesn't generate thrust. It either gets blown around by the wind or needs some other form of engine to propel it horizontally.

@reirab: Haven't you even blown up a balloon, and then let go of the nozzle so it flies around the room?  What causes that, if not thrust?

@jamesqf Ah, I wasn't thinking of that kind of balloon. That kind of balloon doesn't work as a blimp (i.e. doesn't generate lift) unless you fill it with Helium or Hydrogen. I was thinking a hot-air balloon. That kind of balloon is just a small compressed air tank, which doesn't scale well because of the problems mentioned in David Richerby's comment on R.J. Jones' answer.

A deflating helium-filled blimp could have both lift and thrust! :p

@bjb568 Yes, briefly. :)

A typical modern turbofan engine produces about 85% of its thrust from bypass flow, i.e. "compressed air", that never goes through the actual jet core.
However, as others have mentioned, in order to get the compressors spinning and sucking air in to begin with, you still need an energy source of some kind to turn them and, for lack of a better alternative, current designs still rely on jet core flow to turn both sets of turbines and, by extension, their associated compressors.

Where would you get the compressed air from? Jet engines "contain" compressed air because the engine itself compresses it using the energy from burning fuel. Asking if you could power the engine by that compressed air is like asking if you could power a car engine by the rotation of the gearbox: it's confusing cause and effect.

The engine is compressing air to more efficiently burn fuel. If you remove the fuel, there is no reason to compress the air. Other systems that run off of the engine bleed air only do so because it is a convenient source. The 787 uses a bleedless model that runs off of electric power instead of engine bleed air.
Electric airplanes are certainly an option, but a propeller is a better option than a turbine engine. You could however keep the duct around the prop.

In modern aircraft's jet engines (both turbojets and turbofans), compressed air is used:

For starting the engine, spooling it up until the shut-off valve is opened and the fuel starts to enter and burn in the combustion chamber;
For running auxiliary systems, like de-icing and pressurizing equipments;

This is not true for some newer engines like the General Electric's GEnx that use an electric starter or little jet engines (including APUs) and sometimes turboshafts (turboprops).
In aviation it's common to refers to the systems running/producing compressed air as 'pneumatic' systems or 'bleed-air'.
This, for instance, is the part of the overhead panel of a Boeing 737 used to manage pneumatic systems (taken from http://www.b737.org.uk/pneumatics.htm):

No. Air is compressed because we have jetful to run the compressor in the first place.
No fuel, no compressed air (or very very few, from to drag induced by windmilling the engine).
Have a look at https://en.wikipedia.org/wiki/Jet_engine for more informations
A plane with compressed air tanks would also not be possible, as it requires A LOT of air to move an aircraft

It would be possible. Just not practical. :) Lots of things are possible. Physicists worry about those. Far fewer things are practical and those are what engineers are concerned with.

@reirab i would like the mathematical proof :)

No need for a mathematical proof when we've had working models since at least the 20s. The problem is that it doesn't scale up to anything useful. It works fine for very light R/C planes, though.

@reirab That's the point, theses kind of things might not scale up. But if you consider the R/C as genuine planes I guess you are right :)


text:
What are good tips for flying across Tejon pass?
I will be flying from Northern California to Southern California this weekend for the first time in a C172SP (KPAO to KFUL). My planned altitude is 9500 feet. I had a talked with my former instructor about this flight. We had gone over the flight plan and he told me his tips on flying this route including using extreme caution when flying over the Tejon pass (by Gorman VOR) because of its strong turbulence.
Has any of you flown this path before can possibly give me some tips? Anything is appreciated especially tips about passing the Tejon pass.

Not flown that pass, but have flown banning pass out to Palm Springs, and I can honestly say it was the scariest experience I've had in a Cessna :)

In fair weather and light winds it will be a non-event. Your instructor will, or should, know what that threshold is. As far as passes go, Tejon does not appear to be a super technical obstacle. Be aware of the winds, review ridge crossing guidance, do not expect a 172SP to perform like a rocketship, and above all do not be afraid to turn around and land if you have doubts about the flight. Don't worry...you'll be fine. Enjoy the scenery.

@acpilot Thank you for the info! I'll especially keep this in mind "above all do not be afraid to turn around and land if you have doubts about the flight"

Also, don't round out too soon.

Hey all, thanks for all the great advice. The flight went well. I felt a few bumps here and there at 9500 over the pass and on the way back at 8500 but nothing serious. When I descended above LA basin it was a lot more bumpy than over the pass ;-) Thanks again.

I cross that pass on my commute twice a week in my VariEze at altitudes between 5500 and 11500 ft depending on winds and clouds. There's nothing to be particularly nervous about, there's rarely any noticeable turbulence if you're at 7500 or higher.  You can check the Sandberg AWOS and forecast winds aloft, and ask L.A. Center or Bakersfield Approach for ride reports.
I do tend to be flying that route either fairly early in the morning or around dusk, so it may be bumpy more often in the heat of the early afternoon.
If clouds are forcing you down to 5500 be sure not to accidentally fly up the wrong valley when headed north!

Thanks for the advice! When you said "be sure not to accidentally fly up the wrong valley when headed north!", did you mean the wrong valley as the one leads to Mt Pinos toward the West if I am flying North?

@RoundOutTooSoon Yep, that's the one.  I got a couple of miles up there one scuddy morning before realizing "Hey, that's not I-5!"

Also if the weather isn't good and icing prohibits Ifr I just jump over to the coast and fly down the coast. The area around the pass can really ice up in the winter.


text:
Did the 1500 hour rule expire?
So the 1500 hour legislation was set to expire on the 30th of September 2015. Anyone know the outcome of the expiration. Did congress extend the rule, or is it no longer in effect. I tried searching the web but couldn't find any info.

Do you have a link or citation? I'm not sure that's the correct interpretation of the situation.

This is where I saw the 9/30 date listed http://www.usatoday.com/story/news/2015/08/27/pilot-shortage-faa-1500-hours-required-colgan-crash-alpa/32008451/

What is the 1500 hours rule? Is it country specific?

The 1500 hour rule is still in effect.
The rule wasn't set to expire on the 30th of September; technically the entire FAA was going to expire. Congress votes on the authorization for the FAA to regulate air transportation; that authorization was set to expire 30 September, 2015.
The House voted to authorize funding on 28 September while the Senate passed a 6-month extension on 29 September.
Expect to see similar discussions in 6 months.

Ah! Thanks for the clarification

@egid : "technically the entire FAA was going to expire" ?  Who would take over regulating air transportation?

@user6035379 no one. In the US, An agency being authorized to exist and perform their functions lasts only as long as their funding does. Laws funding agencies are typically only for a small number of years at a time. If the law expires the agency shuts down. Sometimes this happens to the whole government and it is a mess.

@BSteinhurst : I don't understand.  No funding for the FAA.  Then what happens to aviation?  People stop flying?

@user6035379 traffic control are probably one of the essential services that keep going so flying is ok. But the regulators, the folks who make the rules and generally anyone who works in an office would be prevented from working. That is why not passing budgets until the last minute is a great American pastime.


text:
Is it legal to use handheld radios to listen to airport frequencies in Australia?
I am a student pilot at ABX (YMAY) in Australia. It is a single engine aerodrome with three frequencies:

123.25;
125.80; and
the ATIS frequency

I live very close to the airport, directly under the base/final turn. Would I be able to purchase a handheld radio that I could use to listen to the ATC and practise my radio calls? Is it legal?

ICOM makes scanners that can receive that band, and they seem to sell them in Australia: http://www.icom.net.au/receiver_ic-rx7.html. May not be the cheapest option, but seems  to answer the "can I" question.

You should make basic research before considering asking on this website. It would have saved the first part of your question (possibility of purchasing), and perhaps the second part (legality explained in the notice or other documents provided with such purchase)

Not a direct answer to your question, but take a look at liveatc.net

For the questions about legality, range etc. you might try posting on ham.SE too, because those seem like general questions about radio use in Australia.

You can use any basic scanner that includes the "aviation band" (AM) and has a squelch. As this is a receiver only, you won't have any legality question to solve. Icom and Yaesu are indeed good brands for radio hardware, but this is not required for such short distance.

@mins -- this is not necessarily true. The standards in the US might be different than those in Australia. There is one case where Icom has released a product in Australia that does not conform with the relevant Australian standard: http://www.acma.gov.au/theACMA/engage-blogs/engage-blogs/Spectrum/Supporting-aviation-communications-and-operations   So while all handhelds that operate on the frequency would probably work, they might not be legal.

@CJBS: Not compliant with all requirements doesn't tell much about the problem. But in general Icom a renowned brand. According to Wikipedia, it's legal to listen to any frequency in Australia, but private phone conversations.

@mins I'm not sure of the exact details -- the link referred to some minor interference problem. My point is that it might not be legal to just buy a handheld receiver in the US and then use it in Australia, even though it would work (as the frequency bands are the same).

I don't see where the US are mentioned in the question. It looks like the OP lives in Australia.

An Aeronautical License issued by the Australian Communications and Media Authority (ACMA) is required in order to operate a handheld aviation-band VHF transceiver in Australia.

An aeronautical licence is issued to authorise a station that:

is not fixed to an aircraft;
is operated on aeronautical frequencies;
is operated for purposes relating to the operation of an aircraft, or
  airport or aerodrome operations; and
in relation to an aircraft
  includes a mobile station operated on board the aircraft or on the
  ground in communication with aircraft.

An aeronautical licence is
  necessary to authorise the operation of stations providing:

air traffic control services;
aerodrome radio information services;
private company radiocommunications; and
other airport or aerodrome
  services.

Reference: ACMA
The Australian-based recreationalflying.com tutorial website may also provide more useful information.
(Note: This applies for a transceiver -- i.e. transmit & receive -- rules may be different for receive-only).

Receiving seems legal. Indeed transmitting (or owning a transmitter) on aviation frequencies is not permitted, unless you are authorized.

I'm sure it would be legal in Australia to buy a scanner to listen to the aviation band. If you want one that transmits you will probably need a license to operate it if not buy it, but then you really don't want to be transmitting anyway.
Personally I would not think it's worth the money. Listening can be interesting but it gets old quickly and you don't learn as much as you might expect - RT work makes more sense in context. My personal experience is that the hard part of RT is not knowing what to say but getting it out in the cockpit when you are also flying and navigating as the same time, and that only comes with experience.
If you are taking lessons why not ask if you can borrow a handheld radio from your training organization? They probably have one or two lying around.

I'm sure it would be legal in Australia, what makes you say that?

Scanners are legal in most democracies, sometimes with a permit. They are for sale in australian pilot stores and ebay as well, so the balance of evidence would indicate it's legal to buy them.

@GdB I'm aware, but they are illegal in Germany and the UK. I think they're the only ones, however, it's still legal for pilots to buy secondary radios for use in flight, which is why you find them in pilot stores there as well. Especially given the heavy UK influences in Australia, it is a valid concern and I believe it warrants backing up the claim. I know LiveATC is broadcasting from all states in Australia, so I'm pretty sure you're correct, but still.

I'm not sure about Germany but scanners are legal in the UK.

I believe Germany and the UK have similar laws (I'm guessing you'll find them among the wiretapping laws), in that the devices as such are not illegal, but it's illegal to listen to air traffic frequencies unless you're involved in... well.. air traffic, and disseminate any information received to third party. They're idiotic laws, no doubt, that's not the point I'm trying to argue.

I live in the UK and I know of no laws or regulations which would prevent you from buying and using an air band scanner.

I quick Google search tells me it's called the Wireless Telegraphy Act, don't know the letter of the law, but most hits starts out by saying it's illegal to listen unless you're licensed to do so. Don't know what constitutes a license.


text:
Why does cabin air have to be dehumidified?
When air is piped in from outside and into a pressurized cabin, it is also dehumidified in the process.  All airlines provide beverage service on even short-haul flights to combat the dryness of the air.  Why is it necessary to dehumidify cabin air?

A lot of consideration is placed on the airframe (which has to hold for 20+ years) rather than the passengers.
One of the primary reasons is to slow corrosion of the  aluminium airframe. This is also why the Boeing 787 is a lot more humid - corrosion is no longer an issue in a composite fuselage (or a lot less at least).
Boeing and the Economist sum it up nicely I think:

the greatest issue lies with the cold fuselage skin interacting with
  moist air, which will condensate and speed up the rate of corrosion.

But condensation in the gap between cabin and hull can be lethal.
  This gap contains much of an aircraft's wiring, and water can damage
  that wiring's insulation. Such a problem is thought to have
  contributed to the loss of a Swissair DC-11 off the Atlantic coast of
  America a few years ago. Too much condensation causes other
  difficulties, as well. There have been cases when ice has built up
  inside the tailplanes of aircraft, causing their rudders to freeze and
  thus preventing their pilots from steering them.

An aircraft such as a Boeing 747-400 can accumulate as much as 700kg of condensation before it reaches equilibrium. Even though
  that is only about 0.17% of the 400 tonnes such an aircraft weighs
  fully laden, it is enough to cause problems for the pilot when he
  tries to trim the balance of the aircraft. And 700kg is about the
  weight of nine men. So the aircraft is carrying the equivalent of at
  least nine non-fare-paying passengers, in an industry where every
  extra kilogram affects fuel consumption and profit margins.

Dear Boeing and the Economist, "condense" is already a verb. You don't need to add "-ate" on the end to... make it into a verb. In fact, "condensate" is a noun: it means "the stuff that has condensed". In everyday English, that's called "condensation" but, technically, condensation is the process of condensate condensing.

Rather than going to great lengths to dehumidify the cabin, wouldn't it be more cost-effective to simply convince the condensate to pay its travel fares?

Wow, 700 kg! That the typical thing no one cares about... except you are an aeronautical engineer...

Condensationalize perhaps?

Aircraft which operate in high humidity, low maintenance areas like Africa are known to carry up to two tons of water in the fuselage when they arrive at Airbus in Hamburg for a check-up.

I question the Economist article claiming 700kg of condensation.  The Boeing article cited mentions 36kg in a 737-300; although a smaller aircraft, that is an order of magnitude smaller that the Economist's claim.  Also, the Boeing article says it is trapped in the insulative blanket, so weight and balance will be N/A due to the moisture accumulating throughout the length of the jet

FlightGlobal quotes 200kg of extra weight on a narrow-body due to insulation material soaking up water. It looks like there is a broad range of values; it likely depends on where the plane is operated.

There are a couple of things that contribute to the dryness of cabin air:

Cabin air is drawn from engine bleed taps in the high pressure compressor.  This is ambient air, which at the flight levels is normally quite dry and then heated and compressed, which will lower the relative humidity, making it seem even drier.
The air cycle machine subsequently extracts moisture from this hot, high pressure air within the pack and used it to provide evaoprative cooling in part of the pack responsible for cooling the air flowing through it (The pack needs to extract heat so when air is expanded into the cabin it is cool enough - it does this with compression/expansion cycles and cooling).
As the air coming out of the packs undergoes expansion to cabin pressure, it will cool down and this will help counteract some of the dry feeling by boosting relative humidity.

The primary reason low humidity is desirable in most airplanes is that water contributes to corrosion and that is bad for the airframe.
The Boeing 787 takes a new approach and provides pressurization and air conditioning with electric compressor motors instead of engine bleed air, and using this method (and a composite airframe) allows for much higher relative humidity in the cabin.

That bleed air is heated (then cooled back to something people can safely breathe) won't change it's humidity levels much.  It's so dry because it's basically outside air, which at 40000 ft is so cold it contains virtually no humidity.

When I flew the C-141 and C-17, their environmental systems weren't intentionally dehumidifiers (designed to reduce the overall humidity), but they did push the air through a fabric bag called the 'sock' to capture droplets that formed during cooling.  As air continued to flow, that moisture could evaporate and rejoin the stream, or be vented overboard through a drain.
At altitude, the air is very dry.  Flying through weather in the approach/departure environment could cause a lot of moisture to get ingested, and so needed to be sorted out.

To add to the answers above: the other main reason (besides condensation) for keeping humidity low is to prevent the growth of mold and bacteria in the cabin and the air conditioning system, which over time may pose a health hazard to the plane's occupants.
And I can't stress enough that the air is not de-humidified, but that the outside air (which is pumped into the fuselage to provide pressure and fresh air) at cruising altitudes is bone-dry. The air in the cabin is actually more humid than it would otherwise be, thanks to the passengers (when they exhale or sweat they release water vapor into the air, and since about 50% of the cabin air is re-used it helps to keep the humidity levels a little higher than they would normally be).
The newest-generation of widebody planes from Boeing and Airbus (787 and A350) actively humidify the air in the cabin, but that requires carrying extra water on every flight (50-100kg, according to Flight Global, plus the weight of the humidification equipment). Additionally, these airplanes need to install active drying systems in the the areas outside the cabin (e.g. behind the lining or above the cabin ceiling) to prevent the insulation mats from soaking up all that extra humidity (example from CTT). In conclusion, the air will be more humid on these aircraft, but it takes a lot of effort (and weight).


text:
Partial ownership: are there any pitfalls to owning the plane through a shell company?
I'm currently saving money for a plane and training, and the expectations are that I won't have the money for full ownership of much else north of a Cessna 150.  I'm investigating partial ownership instead, and I've read that forming a company/partnership to own the plane is better than each member of the group just pitching in out of pocket.
Are there any drawbacks to using this method for plane ownership?

The main thing to do is to realize why you're creating the corporation.  We created a 501(c)7 (in the US) corporation which is a tax-exempt organization and falls under "social club."  We did this to limit our personal liability.  However, if you are creating the corporation to limit personal liability, you have to make sure to operate like a corporation - call each other "co-owners" instead of "partners" (there are legal ramifications to this, not just semantics).  People around the airport should know it's a co-owned plane by a corporation, not that it's "your plane."  Basically if you want the liability protection of a corporation, you must act like one.
Great book to read regarding the legal side of things is "Practical Aviation Law" by J. Scott Hamilton
Depending on how you set it up (we did a 501(c)7 tax exempt, and yes, you still have to file a tax return), how you use the airplane matters too.  For example, we can't earn money using the airplane (conducting flight instruction to non-co-owners, for example), due to the 501(c)7 status.  Under other corporate structures you can.
Finally, I guess to actually answer your questions: There are some pitfalls, but I think they can be avoided.  Most group ownership things fail because of lack of communication and documentation, forming a corporation kind of forces you to spell out things explicitly and then all involved have no excuse for not knowing "the rules." (You have to write up bylaws and have at least one annual meeting, which forces the communication aspect!)

I was in a club like that.  The big advantage (for us) was that we didn't have to pay sales tax when buying or selling a share.  The club owns the plane (& the tools & the engine reserve, etc) and you buy shares in the club.
We had to have an annual meeting, officers, & file with the state.  It wasn't a big deal.


text:
Why are there so many ADS-B ground towers in western Colorado?
I was reading up on Appaero's new ADS-B OUT transponder and clicked over to the Stratus coverage page for some reason. Then I glanced at the map, and noticed something interesting:

Western Colorado is, well, ADS-B tower central. What gives? Chicago and the northeast are close, but not nearly as dense.

Denver is in the eastern half of Colorado, that cluster is in the rockies. Line-of-sight considerations? (for approaches into Denver perhaps?)

That cluster is centered near the San Juan mountain range, not near Denver at all. There aren't any major airports there, Montrose is the closest thing and that regional.

As GdD commented, the cluster you noticed is not near Denver, but Montrose, CO. The reason for this cluster is Wide Area Multilateration (WAM) with ADS-B. This technique uses many sensors distributed around an area to provide better coverage in areas with mountainous terrain that blocks a single radar or ADS-B station. Western Colorado is one such area that experiences delays when ATC does not have sufficient radar coverage to support operations in inclement weather. Montrose is one of the areas that the FAA is using to develop this technology. This "constellation" serves the following airports:

Gunnison-Crested Butte Regional (GUC)
Montrose Regional (MTJ)
Telluride Regional (TEX)
Durango-La Plata County (DRO)

See also this technical presentation about the WAM project, and the lease for the tower sites.
Also, somewhat related, see this NTSB safety recommendation regarding ADS-B coverage serving SAR needs after a crash near Montrose.

Off-topic; being from Dallas, I find it somewhat disappointing that the airport code TEX isn't in Texas.